Regulation of insulin secretion, glucokinase gene transcription and beta cell proliferation by adipocyte-derived Wnt signalling molecules

Abstract

Aims/hypothesis Adipocytes secrete signalling molecules that elicit responses from target cells, including pancreatic beta cells. Wnt signalling molecules have recently been identified as novel adipocyte-derived factors. They also regulate insulin secretion in pancreatic beta cells and the cell cycle. The aim of this study was to investigate the effect of adipocyte-derived Wnt signalling molecules on insulin secretion and beta cell proliferation. Methods Human adipocytes were isolated to generate fat cell-conditioned medium (FCCM). Ins-1 cells were stimulated with FCCM and transiently transfected with reporter genes. Proliferation assays using [ 3 H]thymidine incorporation were carried out in Ins-1 cells and primary islet cells. Insulin secretion from primary islets was assessed by radioimmunoassay. Gene expression in primary islets was assessed by Taqman PCR. Results Treatment with human FCCM increased the transcription of a T cell-specific transcription factor reporter gene (TOPFLASH) in Ins-1 cells (241%, p<0.05). FCCM induced the proliferation of Ins-1 cells (1.8 fold, p<0.05) and primary mouse islet cells (1.6 fold, p<0.05). Antagonizing Wnt signalling with secreted Frizzled-related protein 1 (FRP-1) inhibited the proliferative effect induced by Wnt3a and FCCM on Ins-1 cells by 49 and 41%, respectively. In addition, FCCM led to a twofold (p<0.05) induction of cyclin D1 promoter activity in Ins-1 cells. Furthermore, FCCM stimulated insulin secretion (204% of controls, p>0.05) in primary mouse islets, and this stimulation was inhibited by sFRP-1. At a molecular level, canonical Wnt signalling induced glucokinase gene transcription in a peroxisome proliferator-activated receptor γ-dependent fashion, thereby defining the glucokinase gene as a novel Wnt target gene. Conclusions/interpretation Taken together, these data show that adipocyte-derived Wnt signalling molecules induce beta cell proliferation and insulin secretion in vitro, suggesting a novel mechanism linking obesity to hyperinsulinaemia.

Downregulation of Sfrp5 promotes beta cell proliferation during obesity in the rat

“…There is strong evidence that the Wnt signalling pathway regulates prenatal and postnatal beta cell development in mice as well as glucose sensing in pancreatic beta cells [27,28]. Also, the activation of Wnt signalling in beta cell lines or in isolated islets has been shown to enhance beta cell proliferation [10,[29][30][31]. Thus, increased β-catenin levels in islets caused an expansion of beta cell mass, whereas the depletion of TCF7L2 reduced proliferation in human islets [10,30].…”

Wnt4 antagonises Wnt3a mediated increases in growth and glucose stimulated insulin secretion in the pancreatic beta-cell line, INS-1

“…Subsequent studies implicated β-cell dysfunction as the underlying cause of diabetes in patients with the TCF7L2 risk variant [2][3][4][5] and have suggested that Wnt signalling plays an important role in both pancreatic development and in the function of mature pancreatic islets [6]. The activation of the canonical Wnt pathway, by ligands such as Wnt3a, has been of particular interest as it can increase β-cell proliferation, decrease apoptosis and improve glucose stimulated insulin secretion [6][7][8][9][10][11][12]. However there is now increasing evidence that non-canonical Wnt ligands such as Wnt4 can antagonise the signalling mediated by Wnt3a and may play an equally important role in β-cell function [13][14][15].…”

Wingless-type MMTV integration site family (WNT) signalling in pancreatic beta cells—more complex than expected

“…The underlying defect is most probably impaired sensing of glucose by the affected beta cells [6]. In line with this, molecular studies indicate co-activation of the gene encoding glucokinase-a major regulator of glucose-sensing in beta-cells-by β-catenin [3]. In addition, knocking down the gene encoding TCF7L2 in adult murine and human islets in vitro reduces beta cell proliferation and insulin secretion [7], emphasising the importance of this transcription factor for beta cell function.…”

“…1a) [2]. WNT-signalling molecules are expressed on murine and human beta cells and WNT signalling has recently been identified as a regulator of beta cell proliferation and insulin secretion in vitro and in animal studies [3,4].…”

“…Krützfeldt and Stoffel suggest WNT4 might protect beta cells from canonical WNT activation by exogenous WNT ligands. Under in vitro conditions, adipocytes can activate WNT signalling in beta cells [3], but it is not proven whether such cross-talk is possible in vivo. Interestingly, WNT-signalling molecules have been detected in the systemic blood circulation in humans and there is evidence that the WNT antagonist dickkopf 1 (DKK-1), which is detectable in peripheral blood, derives from (among other tissues) adipocytes [10].…”